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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Nuclear, Plasma, and Radiological EngineeringCenter for Plasma-Material Interactions
Contact: druzic@illinois.edu
Lithium Walls: The Ultimate Technology for Fusion
D.N. Ruzic, M. A. Jaworski1, T.K. Gray1, V. Surla,
W. Xu, S. Jung, P. Raman
1current address: Princeton Plasma Physics Laboratory
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Outline 2
Introduction
- Why Lithium is so Good – but you all know that already !
Solid/Liquid Lithium Divertor Experiment (SLiDE)
-Experimental Facility
-Results
-Theory
Lithium Molybdenum Infused Trenches (LiMIT)
- How this could work for HT-7 and future devices
Lithium in Divertor Erosion Vapor Shielding Exp. (DEVeX)
Lithium in the Ion-Interaction Experiment (IIAX)
Conclusions
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
No cold hydrogen returns from wall: Plasma stays hot
Courtesy: PPPL
What Very-Low Recycling Does for Fusion
Standard Case
Lithium Case
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Consequences of LithiumIncreased Confinement Time – seen across the world
Higher Temperatures
Suppression of ELMS (for tokamaks)
Control of Density is possible, even with NBI
Lower Z-effective
Less Fuel Dilution (seen on NSTX)
Negative Consequences?Helium pumping? We have shown this! M. Nieto, D.N. Ruzic, W. Olczak, R. Stubbers, “Measurement of Implanted Helium
Particle Transport by a Flowing Liquid Lithium Film”, J. Nucl. Mater., 350 (2006) 101-112.
Power handling? See the rest of this talk!
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
First Evidence? TFTR Supershots (1984)
Courtesy: PPPL
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
What Can Go Wrong with a Lithium Divertor?
Lithium melts at 180 C and evaporates very quickly above 400 C
So, it will have to be used ultimately as a flowing liquid
It is a liquid conductive metal and therefore subject to MHD effects. After all, fusion devices have large circulating currents and high magnetic fields.
So, careful planning is needed. Maybe it’s MHD effects can be utilized?
It has an extremely low density (half of water) and high surface tension (4 times water) and therefore difficult to deal with. It is also highly corrosive to some materials, such as copper.
So, careful engineering is needed and new materials (for fusion) need to be developed
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Outline 7
Introduction
- Why Lithium is so Good – but you all know that already !
Solid/Liquid Lithium Divertor Experiment (SLiDE)
-Experimental Facility
-Results
-Theory
Lithium Molybdenum Infused Trenches (LiMIT)
- How this could work for HT-7 and future devices
Lithium in Divertor Erosion Vapor Shielding Exp. (DEVeX)
Lithium in the Ion-Interaction Experiment (IIAX)
Conclusions
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
CDX-U Results - The Unexpected Happened
QinputSurface tension drives flow
Visible image
Centerstack
Lithium in tray
R. Majeski et al., “Final results from the CDX-U lithium program,” Presentation at 47th Annual Meeting of the Division of Plasma Physics (APS-DPP), Denver, Colorado, October, 2005.
Trying to melt lithium in CDX-U:- 50 MW/m2 heat flux redistributed
from spot heat- No evaporation despite lithium's
tendency to do so (and purpose of e-beam run!)
Why did the lithium melt the entire tray and not evaporate?- First explanation was
thermocapillary phenomena- Temperature dependent surface
tension resulted in flow and strong convection away from hot spot
- If true, will this work in a divertor without over heating the Li ?
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
A Typical Fusion Heat Flux
J.N. Brooks, et al. J. Nucl. Matl. 337-339 (2005) 1053-1057.
Magnetic configuration concentrates power in “diverted” plasma
-Peak heat flux, steady-state typically 5-20 MW/m2
-Radiant heat flux at solar surface is ~63 MW/m2
-Transients can push the peak higherThermally driven phenomena depend on
heat flux gradients, not just peak values
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
SLiDE at Illinois - Overview
E-beam source
Current density profile
Tray10cm
10cm
10cm
25cm
Solid/Liquid Lithium Divertor Experiment (SLiDE)- Produces temperature gradients
with an electron beam- Creates magnetic field with
external magnet system (these tests at normal incidence)
- Measures temperature distribution in tray containing lithium
- Active cooling for steady-state operation
- Camera system monitors surface velocity
Designed, constructed and operated for this work
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Machine LayoutA sheet electron beam hits an
instrumented tray filled with lithium in a magnetic field.
Future versions will allow the tray to tilt so the angle between the heat flux and the field will be like in a tokamak – almost parallel.
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Electron Beam Drawings and Simulations
7 keV, 2 A, 16 G 20 keV, 0.75 A, 1 kGThere are four filaments, each 10cm
long, parallel to eachother
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Electron BeamDesigned to mimic
divertor heat flux- Actual run parameters
shown in table- Operated at 300W in this
set of experiments – capable of 15kW, which is 35 MW/m2 !
- Typical q0 in NSTX is
~10MW/m2
- Typical dq/dx in NSTX is ~100 MW/m2-m . We matched this number.
Line source with gaussian profile
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Tray System OverviewCoolant supplied from
building sources
- Water at 35 psi
- Compressed air at 80 psi
- Steady-state cooling with liquid lithium temperatures and input power range of 50W – 1500W (with stainless steel tray)
28 thermocouples within tray
- 14 positions for heat flux modeling
- 4 external TCs for coolant calorimetry
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Data Acquisition and ControlThermocouple data acquired
via computer system
- LABJACK USB data modules and amplifiers digitize analog signals. LabVIEW VI monitors temperatures in real time
- Control of magnets and eBeam voltage via computer system (filaments on manual)
Post-run analysis handled semi-autonomously
- Analysis program written to reference and analyze data sets
- Output summarizes and formats for easy plotting and further analysis
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Camera SystemMirror used to view lithium
surface- Electron beam occludes
direct view- Mirror requires periodic
cleaning from lithium evaporation
Two cameras used- Low quality webcam for
simple monitoring- High quality, high-definition
camera used for velocity measurement movies
- Both utilize mirror illumination provided by electron beam filaments
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Thermocapillary Flow, Basics
QinputSurface tension drives flow
Other surface tension driven flows
- “Tears” or “legs” in wine glass (concentration gradients from alcohol evaporation)
- Soap boats with detergent droplet (concentration gradient)
Thermocapillary
- “Thermo” meaning temperature
- “Capillary” meaning related to surface tension
- Temperature gradient on surface of a liquid creates surface tension gradients
- Surface tension gradients result in flow generation
Long history of study from mid-19th century
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Thermoelectric MHD, BasicsThermoelectric effect
- Causes thermocouple junction voltages
- Thermoelectric power present in most materials
- Electromotive force generated by temperature gradients
- Requires different material (or TE power) to provide current return path and generate current
Replace one material with a liquid in magnetic field
- TE current and B-field generate Lorentz foce
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Theory: TCMHD and TEMHD● Thermocapillary and TEMHD produce different
flow patterns● TC forces act parallel to surface gradients in surface
tension – force vectors radiate away from heat stripe (induce poloidal flow)
● TEMHD forces are produced in the bulk lithium due to gradients at the lithium-steel interface
● TE current the same process as produces voltages in thermocouple junctions
● Cross-product of JTEMHD and B results in azimuthal flow
● Beam generated forces (JxB) also result in an azimuthal flow (current converges into impact point) – sense of rotation is opposite of TEMHD
● Increased field damps both flows● TCMHD is damped by B-2
● TEMHD is damped as B-1 at high fields (due to thermoelectric propulsive force)
Thermocapillary force vectors.
TEMHD current diagram.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Is it really TEMHD? Qualitative Tests
Magnetic field reversal- Flow direction reverses upon reversal of
field- Flow is consistent and steady in swirl
Flow direction consistent with TEMHD source- Mirror system reverses apparent sense
of rotation- Magnets measured to determine
direction- E-beam and TE have opposite rotation
sensesAddition of insulator halts any
swirling flow- Quartz slides added between tray and
lithium- No flow observed at all in these cases
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Another test: “Spin Down”● Test based on the time required
for the lithium to come to a stop● Viscous damping brings fluid to a
rest without additional forces to maintain flow (seconds)
● Magnetic fields enhance the destruction of kinetic energy and spin down faster (confirmed with a mercury test)
● If thermoelectric currents exist, these will decay at the thermal time constant of the lithium-tray system (minutes)
● Maintaining the magnetic field will sustain the flow, as opposed to damping it
● Turning magnetic field back on after a viscous spin-down should induce motion once more
Test procedure:
1.Obtain steady thermal conditions and lithium flow
2.Shut off beam and magnetic field and measure spin-down time
3.Return system to steady thermal conditions and lithium flow
4.Shut off beam but maintain magnetic field – measure spin-down time
5.Repeat step 2, but turn magnetic field back on after flow comes to rest and look for spin startup
Movie of spin-down test“spinDown.mov”
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Quantitative Analysis: VelocityPurpose: Bring together
thermal and velocity measurements
Velocity measurements based on video analysis- Particles measured on a frame-
by-frame basis- Multiple measurements made
over course of particle visibility- Radial distance also measured
Velocity and radius used to determine most likely velocity at r = 1cm- u(r) ~ r0.5 based on Davidson
swirling flow theory- Consistent with SLiDE data
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Heat Flux CalculationTime series data reduced
- Mean taken- Standard error calculated by
standard formulasFourier model applied to
calculate heat flux- Scaling factor applied to account
for tray warping- Radially symmetric pattern
observed- Most obvious in “off-center” sensor
sets- Radially symmetric pattern
observed in all cases run- Some TC pairs eliminated due to
tray damage directly underneath beam strike
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Quantitative Prediction: TEMHD● Moderate Hartmann number
regime – TEMHD and MHD braking in equilibrium
● Dependent on thermoelectric power of the metal pair, P
● Temperature gradient along the interface determines flow velocity (for Ha > 1)
● Mean current density due to TE currents depends on geometry and conductivities (C variable)
● Velocity prediction can be converted to Reynolds number using depth as the characteristic length scale
1D current driven flow in a semi-infinite domain.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
TEMHD Solution 1: Semi-Infinite DomainSolution for free surface
Identical to Shercliff, 1979 channel flow
Lithium-Iron example: P = 20e-6[V/K], h=5[mm], dT/dy = 1000[K/m]
Pre-factor = 8.4[m/s]
Solution depends on several factors
Hartmann again present
“C” is ratio of liquid/wall impedances
“Pre-factor” and velocity function {...}
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Bödewadt-Hartmann FlowBödewadt flow
- Rotating fluid over stationary disk
- Variation of Karman flow (rotating disk)
- Use Karman similarity variables to analyze
Bödewadt-Hartmann flow
- Rotating flow with a magnetic field
- Non-dimensionalized system of equations results
Elsässer No. - Balance of MHD to Coriolis force
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Approximate Solutions for Velocity Profile
Distance from wall
Make use of Davidson, 2002 approximate solutions
Linearized equations about core flow solution
Compares well with direct numerical integration
Aids further analysis
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Quantitative Assessment --- Consistent with Data
Theory of TEMHD driven swirling flow predicts radial velocities -- agrees with data ! No free parameters
Range of Hartmann number covers peaking area of swirling flow theory
Range of Elsässer number covers MHD and Coriolis dominated flows
Torque balance method works ! TEMHD fits observables.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Spin Down Time Constant – Also ConsistentSpin down time requires
thermal gradients to sustain currents
Thermal time constant of the system can be estimated- Simple thermal resistance
model applied- Measurements from the
system used to make calculation
- 78 seconds is theoretical thermal time constant
Observed spin down time is equivalent to 2-3 thermal time constants – and that is what is observed.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Quantitative Prediction: Thermocapillary MHDConsider a semi-infinite
domain
- Free-surface at y=h
- Magnetic field B
- Surface subject to constant temperature gradient b
Two cases
- No return flow (dP/dx = 0)
- Return flow (dP/dx related to height of the fluid)
Surface tension boundary condition
- Surface tension gradient results in viscous shear at surface
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Which one does theory say is dominant?Ratio of Semi-Infinite Solutions
Ratio of TEMHD to TCMHD velocity:
- Ratio = 1 indicates equal effectiveness
- Ratio > 1 indicates TEMHD dominance
- Ratio depends on material parameters capture by dimensionless number, ς, the “Jaworski” number.
- Also depends on container geometry captured in F(Ha) function
In Lithium-steel system, TEMHD dominates for Ha > 1
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Ratio of TEMHD to TC in SLiDEAll quantitative data cases show
evidence of swirling flow.
- TEMHD indirectly shown for Ha>1.4 by temperature
- TEMHD directly shown for Ha>17
However, TC was capable of being seen in an oscillatory flow behavior.
- TEMHD flow distributes heat and smooths out the gradient along the Li – steel interface
- With no gradient there, only the surface temperature gradient exists, therefore TCMHD until interface gradient builds up again
Jaworski Number was always greater than 1 !
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Can you see TCMHD in the lulls?
If conditions are right, when the TEMHD flow stops, TCMHD “Maragoni-effect” flow can be seen (motion on surface away from heated stripe due to surface temperature gradient causing a surface tension gradient)
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If conditions are right, when the TEMHD flow stops, TCMHD “Maragoni-effect” flow can be seen (motion on surface away from heated stripe due to surface temperature gradient causing a surface tension gradient)
TCflowExample.mov
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Outline 38
Introduction
- Why Lithium is so Good – but you all know that already !
Solid/Liquid Lithium Divertor Experiment (SLiDE)
-Experimental Facility
-Results
-Theory
Lithium Molybdenum Infused Trenches (LiMIT)
- How this could work for HT-7 and future devices
Lithium in Divertor Erosion Vapor Shielding Exp. (DEVeX)
Lithium in the Ion-Interaction Experiment (IIAX)
Conclusions
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Apparatus for Seebeck Coefficient
HEATER
T T + T
V
Lithium Extrusion
39How Powerful is the ThermoElectric Effect?
Apparatus for Seebeck
Coefficient Measurement
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Big! Use it with what holds the Lithium
Like a thermocouple, a voltage is created at a junction of two metals dependent on the temperature.[1]
A current will flow based on that voltage difference:
where σ is the conductivity, and ΔS is the difference in Seebeck coefficients.[2]
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Lithium
MolybdenumTSj
j
j
There is a large difference in S between Li and most other metals and it increases with temperature.[3]
0 20 40 60 80 100 120 140 160 180 200 2200
10
20
30
40
Experimental Data Bidwell (1924)
Se
eb
eck
Co
effi
cie
nt (
V
/K)
Temperature ( oC)
Seebeck Coefficient of LiBlack: our measurements Lithium
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Does it work? Yes! SLiDE ResultsAs I showed previously, TEMHD is real and
moves Lithium at significant velocities.
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Theoretical vs experimental velocities :
[4] M.A. Jaworski, et al. Phys. Rev. Lett. 104, 094503 (2010)
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Poloidal limiter
Toroidal limiter
Belt limiter
The Idea: “LiMIT” Design
Left is a cross-section of HT-7 showing the toroidal limiter.
Right is the LiMIT concept: molybdenum tiles with radial trenches containing lithium. The trenches run in the radial (polodial) direction such that they lie primarily perpendicular to the torroidal magnetic field.
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HT-7 Cross-section:
Plasma primary heat-flux location
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Lithium Flow in the Trenches is Self-Pumping
Concept for heat removal using TEMHD. The Li flows in the slots of the Mo plate powered by the vertical temperature gradient. This vertical temperature gradient generates vertical current, which when “crossed” by the torroidal magnetic field, will create a radial force on the Li driving it along the slot. This flow will transfer the heat from the strike point to other portions of the torroidal limiter. The bulk of the Mo plate could be actively cooled for a long-pulsed device or passively cooled for something like NSTX. Under the plate the Li flows back naturally.
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The top surface of the Li is hotter than the surface that is deeper. Therefore there is a VERTICAL temperature gradient Hot
Heat flux
Cooling channelsInlet
Outlet
Passive Li replenishment
Li flow BJ
F
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Thermoelectric Driven Flow Calculation
Look just at one channel. All channels are equivalent
The width of Li (w) is 1mm and the width of the Mo (t) trench is 1mm. The depth of Li (h) is 5mm. The length of the Li (L) is 100mm. The magnetic field is 2T. The heat flux on top surface is
The unit for heat flux is MW/m2 and the unit for x is m. (based on the formal HT-7 heat flux data)
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Heat flux: divertor strike point width, 1cm
tw
h Grad T TE current Flow
L
B
Z direction
This represents the divertor heat flux maximum value for the calculations here, but the concept should work for even higher values..
0.07x0.03 when 6
0.1x0.07 and 0.03x0 when 3/2605.03/400 x
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Heat flux on the limiter (along radius direction)
The heat flux figure is from F. Gao et al. Fusion Engineering and Design 83 (2008) 1–5
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0.00 0.02 0.04 0.06 0.08 0.100
2
4
6
8
Hea
t flux
(M
W/m
2)
Distance along the width of limiter (m)
Top heat flux profile
0.07x0.03 when 6
0.1x0.07 and 0.03x0 when 3/2605.03/400 x
6 MW/m2 on our geometrical segment is 960 W
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Heat load on LiMIT: What Velocity is Needed?46
To see if a flowing surface can remove that much power with a lower temperature rise, we can use the following equation [5] :
Here the first term is the heat conduction term and the second is the heat convection which transfers the heat along the trench. ΔT+ is the temperature difference across the depth of Li and ΔTll is the temperature difference between the inlet and outlet of the Li trench. When using the HT-7 heat flux data, and both temperature differences
are assumed to be 200K, the velocity needed is u=35.2cm/s. With this flow the temperature rise would only be 200K.
The assumed heat flux means the 1mm Li slot needs to handle 960W heat load.
To put this in the proper perspective, the temperature rise over the 5 second shot of an uncooled one-half inch thick (1.2cm) Mo plate the same size as our imagined Li/Mo trench/finger can be calculated using as follows: E = 960W*5s = 4800 J. V = (0.2cm)(1.2cm)(10cm) = 2.4 cm3 Cv=2.57 J cm-3 K-1 for Mo. Therefore the temperature rise for a passive plate is 778K.
TVcE v
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
What Velocity is Generated by TEMHD ? 47
The Seebeck coefficient for Mo is about 13μV/K (at 400C). The value for Li reported in the literature and measured ourselves is 43μV/K (at 400Cfor a difference of 30μV/K
The current and velocity can be calculated with these two equations
[1]
[1]
Here P is the difference of Seebeck coefficient (thermoelectric power) between two material. Ha is the Hartmann number . C is a parameter coming from the conservation of current:
The velocity with a higher thermoelectric power is 59.6 cm/s. This is large enough to carry away the heat and prevent the Li from getting more than 200C hotter than when it came in.
dz
dTP
CuB
C
Cjs
1
1
1
)tanh(
)tanh(**
HaCHa
HaHa
dz
dT
B
Pu
wBHa
Mo
Li
t
w
C
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Further Analysis: 3D – FLUENT Calculation
What will really happen to the temperature of the Li when the strike point heat flux hits LiMIT? Is the 200K gradient assumption reasonable? A simple 3D heat transfer model is run with FLUENT.
FLUENT has convection flow and 3D heat transfer included.
Boundary Conditions: The Li flows through the slot between two Mo plate and flow back below the Li slot. The inlet velocity is set to be 60 cm/s. The initial temperature is 470K. The top heat flux is that on HT-7. The bottom temperature is set to be 470K as a constant. Will we remain under 670K as the one-D calculation indicated?
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Mo plate
6MW/m2 heat fluxLi channel
Back flow of Li
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Answer: Yes !!!!
Left figure is the temperature distribution of the top surface of Li slot and right figure is the temperature change along the center line of the top surface.
The temperature difference between the inlet and the outlet is about 200K– great! The highest temperature is 691K (418C). There will be evaporation there – which gives additional cooling not included in the model. Some evaporation (radiative cooling) is acceptable and even desired.
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0.00 0.02 0.04 0.06 0.08 0.10450
500
550
600
650
700
750
Tem
pera
ture
(K
)
Position along the trench (m)
Temperature along the center line of the top surface
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Does the Radial Temp. Gradient Cause Ejection?
One concern about using free surface Li is the ejection problem. The temperature gradient along the Li flowing direction will generate a thermoelectric current along the same direction and the Lorentz force may could eject the Li into the plasma. Fortunately the primary gradient pushes the Li downward.
On the inboard side, the force is upward. Similar to the capillary porous system (CPS) [6] which effectively has very narrow channels, LiMIT’s trench design has very narrow slots (1 mm) to utilize the capillary force to hold the Li in place.
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Li
Mo
Heat flux
Grad T TE current
Lorentz Force
B
0.00 0.02 0.04 0.06 0.08 0.10450
500
550
600
650
700
750
Tem
pera
ture
(K
)
Position along the trench (m)
Temperature along the center line of the top surface
F
FJ
J
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Does it Work? Capillary Force Balance 51
The thermoelectric current parallel to the Li flowing direction is [1]. Here the temperature difference is also assumed to be 200K and the temperature gradient dT/dy=2000K/m. Under these conditions jTEMHD=1.78*105A/m2. So total current along the Li trench is 0.89A. But only about 2cm Li will provide a upward force and the force from the TEMHD is 0.035N.
The capillary force is 2ΣL and Σ=0.3N/m [7] at 600K. So the capillary force is about 0.06N.
Aren’t there radial fields and eddy currents that could make things bad? Sure, but there are also a number of mitigating factors:
The capillary force is not effected by the thermoelectic power, P. It P is actually smaller, the forces will balance. Also, if the trenches were narrower instead of 1mm wide, the force would be higher. A coarse mesh could even be used which would have even higher restorative forces.
The leading edges of the Mo fingers could be Mo-sprayed, greatly increasing the surface area and therefore the capillary force. This would easily hold the Li in too.
Also, the part of the Li with the highest ejection force is also the hottest Li and has likely already evaporated anyway!
dy
dTP
CjTEMHD
1
1
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Can Also Work with Porous Material
Utilize porous material infused with liquid metal
- Porous material structure creates TE loops with liquid metal
- Small pores create strong capillary forces at free-surface (similar to CPS)
- Pore size can be engineered to optimize TEMHD effect
Primary temperature gradient is due to incident heat flux
- Pumps liquid metal radially
- Passive replenishment system for a liquid surface !
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
We will test this: Future Work at Illinois 53
The SLiDE experiment at Illinois is being reconfigured to test this concept. We expect to be able to show radial flows of Li along radial trenches in a Mo plate and measure the flow velocity compared to calculations. An electron beam is used to provide the heat flux while the magnet can generate about 800 Gauss magnetic field parallel to the tray surface. The temperature rise of the Li will also be monitored and compared to theory.
Return channels (lithium hydraulic engineering) will also be tested to find a design compatible with tokamak operation. I have ideas on this for HT-7.
E-beam line source
Li TrayMagnet coil
Heat Stripe for SLiDE’s Electron Beam
B
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Conclusions for LiMITA Mo trench structure with flowing Li is proposed as a
potential method to absorb the high heat flux on the torroidal limiter. The thermoelectric effect is utilized to drive the Li flowing along the radius trench direction.
The heat transfer ability is estimated based on the 1-D heat transfer model and simulated in the 3-D domain. Both give positive results showing the ability of flowing Li to mitigate the peak heat flux and to transfer the heat without an unacceptable temperature increase.
The ejection problem is analyzed and should be able to be suppressed by the capillary force.
A simple trench structure system is under construction at UIUC to validate this design. Use on NSTX is being contemplated. It works for divertor tokamaks too.
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Reference
[1] M.A. Jaworski, Ph. D. thesis, University of Illinois, Urbana, IL (2009)
[2] J.A. Shercliff, Thermoelectric magnetohydrodynamics, J. Fluid Mech. 91, 231 (1979)
[3] P. Ioannides, et al. Journal of Physics E 8, 315 (1975)
[4] M.A. Jaworski, et al. Phys. Rev. Lett. 104, 094503 (2010)
[5] M.A. Jaworski, et al. Journal of Nuclear Materials 390–391, 1055–1058(2009)
[6] V. A. Evtikhin, et al., Fusion Engineering and Design 49–50 (2000) 195–199.
[7] M. A. Abdou,et al. On the exploration of innovative concepts for fusion chamber technology: APEX interim report.282 Technical Report UCLA-ENG-99-206, University of California, Los Angeles, November 1999.
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Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Outline 56
Introduction
- Why Lithium is so Good – but you all know that already !
Solid/Liquid Lithium Divertor Experiment (SLiDE)
-Experimental Facility
-Results
-Theory
Lithium Molybdenum Infused Trenches (LiMIT)
- How this could work for HT-7 and future devices
Lithium in Divertor Erosion Vapor Shielding Exp. (DEVeX)
Lithium in the Ion-Interaction Experiment (IIAX)
Conclusions
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
What is Vapor Shielding?What is Vapor Shielding?
-- High density of eroded material at PFM surface
-- Vapor absorbs a significant fraction of incident plasma energy
-- Reradiates absorbed energy
Should:
-- Reduce PFM surface temperature
-- Therefore reduce amount of material eroded
Effect had not been directly verified experimentally, so we built an experiment at Illinois to investigate the phenomenon
A. Hassanein and I. Konkashbaev. J. Nuc. Mat., 313–316 (2003) 664-669.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Divertor Edge Vapor Shielding Exp. (DEVeX)ESP-gun (2004-2006)
- 500 J Capacitor Bank
- 1017 ≤ ne ≤ 5(10)18 m-3
- Te ~ 50 eV- Edep ~ few Joules
Needed More Edep!
Needed a bigger θ-pinch!
Construction began Fall 2006
Operational Winter 2008Plasma!(with some grounding issues)
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Details of DEVeX
Segmented θ-coil
Target Chamber-10-7 mTorr base pressure
Li Magnetron Transmission plates-1” thick Al plates-15 Coaxial Transmission lines
Cu Flux Conserver
Coil ID 10 cm Peak Current 300 kA
Divergence Angle ~ 1˚ Rise Time 3.5 μs
Length 30 cm Pulse Length ~ 300 μs
Translation Distance 22 cm H2 Pressure1-5 mTorr
(static)
Capacitor Bank-fired by spark gap incurring lots of loss-60kV capacitors, but used at less than 30kV. Ultimate stored energy is 250kJ. We only used up to 20kJ.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Lithium MagnetronUsed to deposit thin layers of
lithium onto stainless steel (SS) target
< 100 nm thick
30 min deposition @ 400 mA
Allows the removal of surface contamination from lithium before deposition
30 min -- 2 hours
@ 100 -- 200 mA
Comparison between bare SS and lithium coated target
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Time Averaged Density of Incident Plasma
Stark broadened Hβ line
Gaussian fit
Hβ line less sensitive to Te
Machine broadening ~ 0.036 nm
Measured widths range from:
Corresponds to
For 20 - 25 kV discharges
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Target Region - Te
Triple probe gives a Te of ~70 eV. This is suspect due to certain aspects of the probe.
Triple probe Te data was an over-estimate and not backed up by target temperature rise.
We estimate more in the range of 10’s of eV for the Te and Ti in our plasma burst.
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Target Region - ne
20 kV → 3 X 1021 m-3
450 μs pulse duration
Noise spikes, at beginning, from θ-coil ringing
Triple probe density data is consistent with Stark broadening
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Thermal response of target plate
TC bead
Top View
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Target Surface Temperature
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Outline 66
Introduction
- Why Lithium is so Good – but you all know that already !
Solid/Liquid Lithium Divertor Experiment (SLiDE)
-Experimental Facility
-Results
-Theory
Lithium Molybdenum Infused Trenches (LiMIT)
- How this could work for HT-7 and future devices
Lithium in Divertor Erosion Vapor Shielding Exp. (DEVeX)
Lithium in the Ion-Interaction Experiment (IIAX)
Conclusions
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Ion Interaction experiment (IIAX)
Colutron Ion Source ( >1014 ions/cm2-sec) for both Gaseous and Metal Species (H+, D+, He+ and Li+)
67
Target holder with a UHV heater
Li evaporator
QCM
Gas IN
Faraday cup
Plasma cup
ExB filter
LiCl holder
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Quartz Crystal Microbalance 68
0 5000 10000 15000 20000 25000 30000
5975900
5976000
5976100
5976200
5976300
5976400400oCFrom X = 1712 to 5117 = -0.00228 Hz/sec
Te
mp
era
ture
( oC)
QC
M F
req
ue
ncy
(H
z)
Time
250
300
350
400
450
500
550
600
650
700
Evaporated/Sputtered particles are detected by QCM.
target
1 1 mY
S m I t
: Sputtering Yield [atoms/ion]: sticking coefficient: fraction reaching to QCM: mass of target material: average current: Mass change on QCM crystal
YStargetm
Imt
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Results: Li on C has suppressed evaporation 69
200 300 400 5001014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
Li/ATJ D-Li/ATJ Li/SS D-Li/SS bulk Li (ref)
Eva
po
ratio
n F
lux
(#/m
^2/s
)
Surface Temperature (oC)
Evaporation fluxes from intercalated Li are suppressed.
D-saturation also suppresses evaporation fluxes.
Evaporation flux of Li
Vapor pressure of Li
Lithium at Illinois, Hefei, ChinaAugust 20, 2010
Conclusions: Impact on the Fusion CommunityA flowing lithium divertor could be pumped by the very
heat flux it is supposed to remove.
Temperature of Li could be kept below the point where it significantly evaporates
Vapor shielding suggests thin lithium films absorbs large fraction of energy deposited during an ELM or disruption
-- Could “save” the PFCs from large temperatures that would otherwise be encountered possibly damaging the underlaying structure behind the lithium PFC.
Sputtering of Li is suppressed by impurities and by absorbed D.
The future of Lithium is Bright!